The optical-infrared astronomy group conducts observational research on active astronomical phenomena, in particular, so-called “transient objects” including supernova explosions, gamma-ray bursts, and active galactic nuclei, using the Kanata 1.5-m optical-infrared telescope in collaboration with X-ray and gamma-ray astronomical satellites. Here we present a brief summary of our research activities.
What is transient objects?
Stars observed from the ground appear to be very quiet and stable. However, the real universe has a different face, that is dynamic and active, behind its apparently calm mask. For example, even our Sun, which seems to be shining quite stably, is also very active; it changes brightness continuously and repeated explosions occur on its surface, when we observe it in detail. There are many kinds of active (i.e., showing large brightness and color variations over short timescales) astronomical objects such as supernovae, cataclysmic variable stars, gamma-ray bursts, and active galactic nuclei in the universe. Those variable objects are called “transient objects”. Most such objects emit huge amounts of energy in short time periods. They are closely related to the birth, evolution and death of stars and galaxies. Therefore, it is very important to reveal the mechanisms that give rise to the activity and emission of transient objects in order to answer fundamental questions such as “where did the various elements of our world came from”, “how and where was our Sun born”, or “what is the future fate of our Sun”. The optical-infrared astronomy group at CORE-U is conducting intensive studies on transient objects involving multi-wavelength observations centered on the optical-infrared region in order to understand the evolution of stars and galaxies.
Transient objects and the extreme universe
Phenomena related to high-energy activity in the Universe, such as vast explosions associated with the death of stars, energy release from extremely high-density compact objects that are the remnants of dead stars, and relativistic jets from super-massive black holes in the center of galaxies, can be observed as transient objects. In other words, transient objects are phenomena that are so energetic that they cannot be reproduced or tested in any current ground-based experiments. Observational studies of transient objects provide important clues for understanding the physics of such an extreme Universe.
Michitoshi Yoshida (Professor)
There are many galaxies in the Universe. Galaxies are complicated systems containing about a hundred billion stars, a huge amount of interstellar matter and very massive dark matter. The typical size and mass of a galaxy are 100,000 light years and a quadrillion (1012) solar masses. Galaxies are cradles and graves, in which stars are born and die continuously.
The formation and evolution of galaxies have been revealed step by step with modern astronomy, but there are still many unresolved problems. My main research field is an observational study of active galaxies. My goal is to understand the relation between galaxy evolution and galaxy activity. I also am interested in transient objects such as gamma-ray bursts and gravitational wave sources, and am developing instruments for these kind of observations.
Koji Kawabata (Associate Professor)
Although the stars in the night sky seem to shine fairly constantly like the Sun, they undergo births and deaths and can show violent activity, particularly when dying. A supernova is one of the most energetic stellar explosions, in which the entire stellar atmosphere is blown away; it can temporally brighten to billions of times the luminosity of the Sun. In addition, some supernovae are accompanied by very high-energy gamma-ray bursts. Such events have contributed to the current state of the Universe. We are carrying out observations using the 1.5-m Kanata Telescope in Hiroshima University and other telescopes and observing facilities. We are also developing state-of-the art instruments for astronomical observations.
Makoto Uemura (Associate Professor)
Explosions are constantly occurring in the Universe. My research focuses on explosive events that occur during accretion processes involving strong, compact gravitational sources such as black holes. In such systems, the gas forms an “accretion disk” around the compact object. High-speed plasma flows, and so-called “jets” are also occasionally observed. However, we cannot directly observe the structure of such components because they are too far from us. Recently, I have been studying methods of reconstruction of such objects based on their temporal variations.
Yousuke Utsumi (Assistant Professor)
I am a member of the Hiroshima Astrophysical Science Center, and am now conducting the HinOTORI project to build a new 50-cm telescope in Tibet, China. The instrument is a three-color simultaneous imager. The scientific goal is to reveal the nature of a electro-magneto counterpart of the gravitational wave source.
Hiroshi Akitaya (Assistant Professor)
Stars like the Sun are formed through the collapse of molecular clouds composed of gas and dust particles. I am studying the formation mechanism of stars by optical and infrared observations. One of the most important and unique instruments in the world for such research is the Hiroshima optical and near-infrared camera HONIR, which is capable of imaging, spectroscopy, and polarimetric observations, at optical and near-infrared wavelengths simultaneously. I am involved in the development and operation of this instrument.
I am also promoting the Optical and Infrared Synergetic Telescopes for Education and Research (OISTER) project, in which gamma-ray bursts or transient objects are observed using the Kanata telescope in cooperation with a dozen other telescopes in Japanese universities and research facilities, as a representative of Hiroshima University.